It has been known that in the presence of a stereospecific catalyst of various types, a crystalline polymer or copolymer of propylene (the two will sometimes be generically referred to as polypropylene) is produced in a first step, and in a second and a subsequent step, propylene and another alpha-olefin are copolymerized in the presence of the above polypropylene to produce a crystalline polymer or copolymer of propylene and/or the other alpha-olefin, above all a crystalline polymer or copolymer of ethylene or a crystalline polymer or copolymer containing ethylene as a main component. It is known that such a multiplicity of steps give a composition having improved impact strength at low temperatures while retaining the excellent rigidity of polypropylene.
The composition is usually a uniform and intimate mixture of the polymers or copolymers produced in the individual steps, but is generally referred to as a block copolymer. The block copolymer gains widespread use in containers, automobile parts, easily low-temperature sealable films, high impact films, etc.
Japanese Laid-Open Patent Publication No. 115296/75 and its corresponding U.S. Pat. No. 3,974,236 disclose a process for producing a heteroblock copolymer of propylene and ethylene in two stages described below. The first stage consists of at least one step of producing isotactic polypropylene by polymerizing propylene in the presence of a catalyst and at least one step of producing an ethylene/propylene copolymer by polymerizing a mixture of ethylene and propylene in the presence of a catalyst. The ethylene content of this mixture is 0.2 to 3% by weight, and the content of isotactic polypropylene is at most 25% by weight of the heteroblock copolymer produced in the first stage. The first stage is started by polymerizing propylene and the two steps are successively carried out. In the second stage, another copolymer of ethylene and propylene is produced by polymerizing a mixture of ethylene and propylene in the presence of a catalyst until the quantity of the copolymer reaches 5 to 20% by weight of the heteroblock copolymer to be finally produced. The ethylene content of the mixture is at least 50%. The above patent specifications state that the heteroblock copolymer has excellent surface gloss, high flexural rigidity, and good impact strength.
British Patent No. 1,543,096 discloses a process for producing a chemically blended propylene polymer composition suitable for use in producing molded articles having excellent properties such as high impact strength and rigidity.
This process comprises (i) producing crystalline polypropylene component, (I) in a first step wherein propylene optionally containing up to 1 mole% of another olefin is polymerized in the presence of a catalyst composed of (A) a carrier-supported titanium catalyst component containing at least magnesium, halogen and titanium on the surface of the carrier and (B) an organoaluminum compound, (ii) producing in a second step wherein propylene and ethylene are copolymerized in the presence of the reaction product of the first step and the same catalyst while maintaining the content of propylene in the gaseous phase of the polymerization zone at 65 to 90 mole%; and (iii) producing polyethylene or an ethylene/propylene copolymer component (III) in a third step wherein ethylene or both ethylene and propylene are polymerized in the presence of the reaction product of the second step and the same catalyst while maintaining the content of propylene in the gaseous phase of the polymerization zone at 0 to 15 mole%.
British Patent No. 1,566,391 discloses a process for producing a chemically blended propylene polymer composition having impact resistance and being suitable for production of molded articles having excellent properties such as high impact strength and rigidity and improved whitening resistance and gloss.
U.S. Pat. No. 4,547,552 discloses a process for producing a propylene block copolymer composition suitable for producing molded articles having excellent impact strength, especially at low temperatures, and excellent rigidity in a well balanced combination. This process comprises (I) a first stage of polymerizing propylene containing 0 to 5 mole% of another olefin in the presence of a catalyst composed of (A) a solid titanium catalyst component consisting essentially of titanium, magnesium, halogen, and an electron donor, (B) an organoaluminum compound, and (C) an organic silicon compound having an Si-0-C bond or an Si-N-C bond to thereby form a crystalline propylene polymer or copolymer; and (II) a second stage of polymerizing propylene and ethylene, or propylene, ethylene and another olefin in the presence of the reaction product of the first stage and the same catalyst as used in the first stage to form a rubbery propylene copolymer and a crystalline ethylene polymer or copolymer.